Experimental determination of natural convection heat transfer coefficients in an attic shaped enclosure

Heat transfer by natural convection in triangular enclosures is an area of significant importance in applications such as the design of greenhouses, attics and solar water heaters. However, given its significance to these areas it has not been widely examined. In this study, the natural convection heat transfer coefficients for air in an attic shaped enclosure were determined for Grashof Numbers over the range of 10⁷ to 10⁹. It was found that the measured heat transfer coefficients could be predicted to within 5% by Ridouane and Campo's [E.H. Ridouane, A. Campo, Experimental-based correlations for the characterization of free convection of air inside isosceles triangular cavities with variable apex angles, Experimental Heat Transfer 18 (2) (2005) 81–86] equation (Eq. (1)) for natural convection in a triangular enclosure previously developed for Grashof Numbers in the range of 10⁵ to 10⁶.

equation(1)

Nu=0.286A−0.286Gr1/4.$Nu=0.286A^{-0.286}G{r}^{1/4}\text{.}$

Mixed convection in a lid-driven triangular enclosure filled with nanofluids

This paper presents the results of a numerical study on the mixed convection in a lid-driven triangular enclosure filled with a water–Al₂O₃ nanofluid. A comparison study between two different scenarios of upward and downward left sliding walls is presented. The effects of parameters such as Richardson number, solid volume fraction and the direction of the sliding wall motion on the flow and temperature fields as well as the heat transfer rate are examined. The results show that the addition of Al₂O₃ nanoparticles enhances the heat transfer rate for all values of Richardson number and for each direction of the sliding wall motion. However, the downward sliding wall motion results in a stronger flow circulation within the enclosure and hence, a higher heat transfer rate.     

Suppression of natural convection heat transfer coefficients in an attic shaped enclosure

The use of convection suppression devices has been widely discussed in the literature as a means of reducing natural convection heat loss from enclosed spaces. In this study the use of a single baffle was examined as a possible low cost means of suppressing heat loss by natural convection in an attic shaped enclosure.     

Simultaneous mass and heat transfer under mixed convection along a vertical cone

Steady laminar binary mixed convection flow along a vertical circular cone under the combined buoyancy effects of thermal and species diffusion is studied analytically. The analysis is confined to mass diffusion processes with low concentration levels. In the analysis the surface of the cone is assumed to be at a uniform temperature and uniform concentration. Numerical results for the local Sherwood number, local Nusselt number and local friction factor are presented. Representative temperature, concentration and velocity profiles are also shown. The analysis covers the diffusion of common gases and vapours into air. Considerations are given to the situations where the buoyancy forces assist and oppose the forced convection flow for various possible combinations of the thermal and species diffusion processes.     

Cocurrent turbulent mixed convection heat and mass transfer in falling film of water inside a vertical heated tube

A detailed numerical study has been conducted in order to analyse the combined buoyancy effects of thermal and mass diffusion on the turbulent mixed convection tube flows. Numerical results for air-water system are presented under different conditions. A low Reynolds number k-ε turbulent model is used with combined heat and mass transfer analysis in a vertical heated tube. The local heat fluxes, Nusselt and Sherwood numbers are reported to obtain an understanding of the physical phenomena. Predicted results show that a better heat transfer results for a higher gas flow Reynolds number Re, a higher heat flux q_w or a lower inlet water flow Γ₀. Additionally, the results indicate that the convection of heat by the flowing water film becomes the main mechanism for heat removal from the wall.     

Natural convection heat transfer in a circular enclosure

Natural convection heat transfer in a circular enclosure, one half of which was heated and the other half of which was cooled, was investigated experimentally, focusing on the effect of the inclination angle. The experiments were carried out with water. Flow and temperature field were visualized by using the aluminum and liquid-crystal suspension method. The results show that with downward heating the heat transfer coefficient increased as the inclination angle of the boundary between the heating wall and the cooling wall approached the vertical. But with upward heating, the heat transfer coefficient showed minimal change, exhibiting a small peak value when the inclination angle was γ ˜ –45°. The heat transfer coefficient of a flat circular enclosure was estimated from the circular enclosure's heat transfer coefficient. These results can be explained by the obtained flow and temperature fields. © 1999 Scripta Technica, Heat Trans Asian Res, 28(2): 152–163, 1999     

Effects of Lewis number on heat and mass transfer in a triangular cavity

The double-diffusive mixed convection in a right triangular is analyzed by solving the mass, momentum, energy and concentration balance equations. The flow is considered to operate in the laminar regime under steady state conditions. Moreover, Galerkin weighted residuals finite element method is applied to solve the governing equations. The study is performed for different values of Lewis number, Richardson number and the direction of the sliding wall motion. Heat and mass transfer characteristics as streamlines, isotherms, isoconcentration lines, average Nusselt and Sherwood numbers are studied for the aforesaid parameters. It is found that heat transfer decreased by 3.6% and 3.7% as Le increases from 5 to 20 at Ri = 5 for cases 1 and 2 respectively. On the other hand, at the same convective regime, mass transfer rate increased by 32.4% and 38.4% as Le increases from 5 to 20 for cases 1 and 2 respectively. Moreover, the flow, temperature and concentration fields are controlled by the direction of the sliding wall.     

Experimental study of natural convection heat transfer in a semicircular enclosure

An experimental study of natural convection heat transfer in a differentially heated semicircular enclosure was carried out. The flat surface was heated and the radial surface was cooled isothermally. The effects of angle of enclosure inclination on the heat transfer across semicircular regions of several radii were measured for Rayleigh numbers Ra_R ranging from 6.72 × 10⁶ to 2.33 × 10⁸, using water as the working fluid. The angle of inclination varied from −90 degrees to 90 degrees with radii R of 50, 40, and 30 mm. The flow patterns were sketched from the results of a visualization experiment using aluminum powder. The temperature measurements in the enclosure were carried out using liquid crystals and thermocouples. The results indicate that different flow patterns were encountered as the angle of inclination varied, and the heat transfer rate was largely dependent on the flow pattern. In particular, enhanced heat transfer rates can be obtained when plume-like flow occurs along both hot and cold walls in the case of an upward-facing hot wall. Heat transfer for the inclined enclosure can be predicted using the equation for a vertical enclosure presented in this paper. © 1998 Scripta Technica, Inc. Heat Trans Jpn Res, 26(2): 131–142, 1997     

Conjugate-mixed convection heat transfer in a lid-driven enclosure with thick bottom wall

Conjugate heat transfer by mixed convection and conduction in lid-driven enclosures with thick bottom wall has been studied by a numerical method. The enclosure is heated from the bottom wall isothermally. Temperature of the top moving wall, which has constant flow speed, is lower than that of the outside of bottom wall. Vertical walls of the enclosure are adiabatic. Governing parameters are solved for a wide range of Richardson numbers (0.1 ≤ Ri ≤ 10), ratio of height of bottom wall to enclosure height (0.1 ≤ h/H ≤ 0.5) and thermal conductivity ratio (0.01 ≤ λ_f/λ_s ≤ 10). Obtained results showed that heat transfer decreases with increasing of λ_f/λ_s ratio, Richardson number and thickness ratio of the wall. Flow strength is affected for only higher values of λ_f/λ_s ratio.     

Parametric study on mixed convection heat transfer in an inclined arc-shape cavity

This paper presents a parametric study on mixed convection heat transfer in an inclined arc-shape cavity subjected to a moving lid. The governing equations for the inclined arc-shape cavity were derived with the incorporation of inertia and buoyant force terms and solved by using the finite-volume method and numerical grid generation scheme. The parametric study considered three physical parameters including inclination angle, Reynolds number and Grashof number, and explored the effect of these parameters on the flow field and heat transfer characteristics. Computations were conducted for the Reynolds number ranging from 100 to 1500, Grashof number from 10⁵ to 10⁷ and inclination angle from 15⁰ to 60⁰. The numerical results show that the flow pattern becomes inertia-dominant and the strength of the primary vortex generally increases as the Reynlods number increases. As the Grashof number increases, the strength of the inertial-induced vortex decreases and the strength of the buoyancy-induced vortex increases. The strength of the vortexes decreases with the increasing inclination angle and the buoyancy-induced flow becomes more dominant. The average Nusselt number increases as the Grashof number increases for all the inclination angles studied here. The local friction increases with the increasing inclination angle, and becomes significant as the Grashof number increases.     

Control of mixed convection in lid-driven enclosures using conductive triangular fins

Control of mixed convection (combined forced and natural convection) in a lid-driven square cavity is performed using a short triangular conductive fin. A numerical technique is used to simulate the flow and temperature fields. The vertical walls of the cavity are differentially heated. Both the top lid and the bottom wall are adiabatic. The fin is located on one of the motionless walls of the cavity. Three different cases have been studied based on the location of the fin. In this context, Cases I, II and III refer to the fin on the left, bottom and right walls, respectively. Results are presented for +x and −x directions of the top lid in horizontal axis and different Richardson numbers as Ri = 0.1, 1.0 and 10.0. It is observed that the triangular fin is a good control parameter for heat transfer, temperature distribution and flow field.     

Natural convection heat and mass transfer from a sphere in micropolar fluids with constant wall temperature and concentration

This work examines the natural convection heat and mass transfer near a sphere with constant wall temperature and concentration in a micropolar fluid. A coordinate transformation is used to transform the governing equations into nondimensional nonsimilar boundary layer equations and the obtained boundary layer equations are then solved by the cubic spline collocation method. Results for the local Nusselt number and the local Sherwood number are presented as functions of the vortex viscosity parameter, Schmidt number, buoyancy ratio, and Prandtl number. For micropolar fluids, higher viscosity tends to retard the flow and thus decreases the natural convection heat and mass transfer rates from the sphere with constant wall temperature and concentration. Moreover, the natural convection heat and mass transfer rates from a sphere in Newtonian fluids are higher than those in micropolar fluids.     

Natural convection in porous triangular enclosure with a centered conducting body

A numerical work was performed to examine the heat transfer and fluid flow due to natural convection in a porous triangular enclosure with a centered conducting body. The center of the body was located onto the gravity center of the right-angle triangular cavity. The Darcy law model was used to write the governing equations and they were solved using a finite difference method. Results are presented by streamlines, isotherms, mean and local Nusselt numbers for the different parameters such as the Rayleigh number, thermal conductivity ratio, and height and width of the body. It was observed that both height and width of the body and thermal conductivity ratio play an important role on heat and fluid flow inside the cavity.     

Natural convection heat transfer in a nanofluid filled semi-annulus enclosure

To investigate natural convection heat transfer in a semi-annulus enclosure filled with nanofluid, the Control Volume based Finite Element Method (CVFEM) is used. The fluid in the enclosure is Cu–water nanofluid. The inner and outer semi circular walls are maintained at constant temperatures while the two other walls are thermally insulated. The Navier Stokes equations in their vorticity-stream function form are used to simulate the flow pattern and isotherms. The numerical investigation is carried out for different governing parameters namely; the Rayleigh number, nanoparticle volume fraction and the angle of turn for the enclosure. The effective thermal conductivity and viscosity of nanofluid are calculated using the Maxwell–Garnetts (MG) and Brinkman models, respectively. The results reveal that there is an optimum angle of turn in which the average Nusselt number is maximum for each Rayleigh number. Moreover, the angle of turn has an important effect on the streamlines, isotherms and maximum or minimum values of local Nusselt number.     

Three dimensional numerical and experimental study of forced convection heat transfer on solar collector surface

In this study, experimental and three dimensional numerical work was carried out to determine the average heat transfer coefficients for forced convection air flow over a rectangular flat plate. Three dimensional numerical simulations were obtained using a commercial finite volume based fluid dynamics code called Fluent 6.3. The experiments were performed for mass transfer using the naphthalene sublimation technique. The results were presented in terms of heat transfer parameters using the analogy between heat and mass transfer. All the experimental results are correlated within an accuracy of ± 12%.     

Conduction-combined forced and natural convection in lid-driven enclosures divided by a vertical solid partition

Numerical simulations of the conduction-combined forced and natural convection (mixed convection) heat transfer and fluid flow have been performed for 2-D lid-driven square enclosure divided by a partition with a finite thickness and finite conductivity. Left vertical wall of enclosure has two different orientations in positive or negative vertical coordinate. Buoyancy forces are taken into account in the system. Horizontal walls are adiabatic while two vertical walls are maintained isothermal temperature but the temperature of the left moving wall is higher than that of the right stationary wall. Thus, heat transfer regime between moving lid and partition is mixed convection. Conduction occurs along the partition. And, pure natural convection is formed between the partition and the right vertical wall. This investigation covers a wide range of Richardson number which is changed from 0.1 to 10, thermal conductivity ratio varies from 0.001 to 10. It is observed that higher heat transfer was formed for higher Richardson number for upward moving wall for all values of thermal conductivity ratio. When forced convection becomes effective, the orientation of moving lid becomes insignificant. Heat transfer is a decreasing function of increasing thermal conductivity ratio for all cases and Richardson numbers.     

Heat and mass transfer in a flat plate solar solid adsorption refrigeration ice maker

A uniform pressure model is presented to describe the heat and mass transfer in an adsorbent bed for a flat plate solar ice maker. This model accounts for heat and mass transfer in a porous bed in a two-dimensional transient process. An experiment has been conducted to validate this model and the calculated results are in good agreement with experiments. With the help of this model, the transient analysis and performance prediction of an intermittent solar powered solid refrigerator can be presented.     

A numerical study of natural convection heat transfer in a composed thermal diode

The effects of inclination on the steady natural convection local heat transfer characteristics in an air-filled enclosure, which is composed of rectangular and parallelogrammic portions, are studied numerically. In this investigation, two geometrical aspect ratios are introduced: one for a parallelogrammic portion of an enclosure, the other for a rectangular one. The governing equations for a two-dimensional, laminar, natural convection process in an enclosure are discretized by the control volume approach which ensures the conservative characteristics to be satisfied in the calculation domain, and then solved by a modified SIMPLE algorithm. The momentum and energy equations are coupled through the buoyancy term. Computations are carried out for Prandtl number Pr = 1 and Rayleigh number Ra = 2.7 × 10⁸. In order to obtain a greater understanding of the flow and heat transfer behaviors, flow patterns with streamlines and isotherms at different inclination angles are shown. Also, the effects of numbers of installed guide vanes in a composed enclosure are studied to consider the enhancement of heat transfer of the inner diode. © 1999 Scripta Technica, Heat Trans Asian Res, 28(7): 573–582, 1999